introduction: what is cosmic reionization?
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Probing the neutral intergalactic medium during cosmic reionization using the 21cm line of hydrogen KIAA-PKU Summer School, Beijing, China Chris Carilli, NRAO, August 10, 2007. Introduction: What is Cosmic Reionization? Current constraints on the IGM neutral fraction with cosmic epoch - PowerPoint PPT PresentationTRANSCRIPT
Probing the neutral intergalactic medium during cosmic reionization using the 21cm line of hydrogen
KIAA-PKU Summer School, Beijing, China
Chris Carilli, NRAO, August 10, 2007
Introduction: What is Cosmic Reionization?
Current constraints on the IGM neutral fraction with cosmic epoch
Neutral Intergalactic Medium (IGM) – HI 21cm signals
Low frequency telescopes and observational challenges
References
Reionization and HI 21cm studies of the neutral IGM
“Observational constraints on cosmic reionization,” Fan, Carilli, Keating 2006, ARAA, 44, 415
“Cosmology at low frequencies: the 21cm transition and the high redshift universe,” Furlanetto, Oh, Briggs 2006, Phys. Rep., 433, 181
Early structure formation and first light
“The first sources of light and the reionization of the universe,” Barkana & Loeb 2002, Phys.Rep., 349, 125
“The reionization of the universe by the first stars and quasars,” Loeb & Barkana 2002, ARAA, 39, 19
“Observations of the high redshift universe,” Ellis 2007, Saas-Fe advanced course 36
Ionized f(HI) ~ 0
Neutral f(HI) ~ 1
Reionized f(HI) ~ 1e-5
History of Baryons in the Universe
Chris Carilli (NRAO)
Berlin June 29, 2005
WMAP – structure from the big bang
Hubble Space Telescope Realm of the Galaxies
Dark Ages
Twilight Zone
Epoch of Reionization
• Last phase of cosmic evolution to be tested • Bench-mark in cosmic structure formation indicating the first luminous structures
Dark Ages
Twilight Zone
Epoch of Reionization
• Epoch?• Process?• Sources?
Cen 2002
Some basics
• Stellar fusion produces 7e6eV/H atom.
• Reionization requires 13.6eV/H atom
=>Need to process only 1e-5 of baryons through stars to reionize the universe
At z>6:
tuniv ~ 0.55 [(1+z)/10]-3/2Gyr
z > 8: trecomb < tuniv
Age of the Universe IGM recombination time
QuickTime™ and aYUV420 codec decompressor
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Gnedin 03
Reionization: the movie
8Mpc comoving
Barkana and Loeb 2001
Constraint I: Gunn-Peterson Effect
z
Gunn-Peterson Effect toward z~6 SDSS QSOs
Fan et al 2006
Gunn-Peterson limits to f(HI)
• to f(HI) conversion requires ‘clumping factor’
• >>1 for f(HI)>0.001 => low f() diagnostic
• GP => Reionization occurs in ‘twilight zone’, opaque for obs <0.9 m
GP = 2.6e4 f(HI) (1+z)3/2
End of reionization?
f(HI) <1e-4 at z= 5.7
f(HI) >1e-3 at z= 6.3
But…
Contraint II: The CMB
Temperature fluctuations due to density inhomogeneities at the surface of last scattering (z ~ 1000)
Angular power spectrum ~ variance on given angular scale ~ square of visibility function
Baryon Acoustic Oscillations: Sound horizon at recombination
Sachs-Wolfe
Thomson scatting during reionization (z~10)
Acoustics peaks are ‘fuzzed-out’ during reionization.
Problem: degenerate with intrinsic amplitude of the anisotropies.
Reionization and the CMB
Constraint II: CMB large scale polarization -- Thomson scattering during reionization
Scattering CMB local quadrapole => polarized
Large scale: horizon scale at reioniz ~ 10’s deg
Signal is weak:
TE ~ 10% TT
EE ~ 1% TT
e = 0.084 +/- 0.016
~ L/mfp ~ Lne e (1+z)2
Hinshaw et al 2008
Constraint II: CMB large scale polarization -- Thomson scattering during reionization
Rules-out high ionization fraction at z> 15
Allows for finite (~0.2) ionization to high z
Most action occurs at z ~ 8 to 14
Dunkley et al. 2008
e = integral measure to recombination=> allows many IGM histories
Combined CMB + GP constraints on reionization
But…
• tuniv = 0.87Gyr• Lbol = 1e14 Lo
• Black hole: ~3 x 109 Mo (Willot etal.)• Gunn Peterson trough (Fan etal.)
Pushing into reionization: QSO 1148+52 at z=6.4
1148+52 z=6.42: Gas detection
Off channelsRms=60uJy
46.6149 GHzCO 3-2
• M(H2) ~ 2e10 Mo
• zhost = 6.419 +/- 0.001
(note: zly = 6.37 +/- 0.04)
VLA
IRAM
VLA
Constrain III: Cosmic Stromgren Sphere
• Accurate zhost from CO: z=6.419 +/- 0.001
• Proximity effect: photons leaking from 6.32<z<6.419
z=6.32White et al. 2003
‘time bounded’ Stromgren sphere: Rphys = 4.7 Mpc
Loeb & Rybicki 2000
‘time bounded’ Stromgren sphere: R = 4.7 Mpc
# ionizing photons = # atoms in volume Luv • tqso = 4/3R3 • <nHI>
=> tqso = 1e5 R3 f(HI)~ 1e7yrs or f(HI) ~ 1 (tqso/1e7 yr)
CSS: Constraints on neutral fraction at z~6
Nine z~6 QSOs with CO or MgII redshifts: <R> = 4.4 Mpc (Wyithe et al. 05; Fan et al. 06; Kurk et al. 07)
GP => f(HI) > 0.001
If f(HI) ~ 0.001, then <tqso> ~ 1e4 yrs – implausibly short given QSO fiducial lifetimes (~1e7 years)?
Probability arguments: f(HI) > 0.05
Wyithe et al. 2005
= tqso/4e7 yrs
90% probability x(HI) > curve
P(>xHI)
Difficulties for Cosmic Stromgren Spheres
(Lidz + 07, Maselli + 07)
Requires sensitive spectra in difficult near-IR band
Sensitive to R: f(HI) R^-3
Clumpy IGM => ragged edges
Pre-QSO reionization due to star forming galaxies, early AGN activity
ESO
OI
Not ‘event’ but complex process, large variance: zreion ~ 6 to 14
Good evidence for qualitative change in nature of IGM at z~6
OI
Saturates, HI distribution function, pre-ionization?
Abundance?
Integral measure?
Local ionization?
Geometry, pre-reionization?
Current probes are all fundamentally limited in diagnostic power
Need more direct probe of process of reionization = HI 21cm line
Local ioniz.?
Low frequency radio astronomy: Most direct probe of the neutral IGM during, and prior to, cosmic reionization, using the redshifted HI 21cm line: z>6 => 100 – 200 MHz
Square Kilometer Array
Advantages to the HI 21cm line
1. Spectral line signal => full three dimensional (3D) diagnostic of structure formation.
2. Direct probe of IGM = dominant component of baryons during reinization/dark ages
3. Hyperfine transition = forbidden (weak) => avoid saturation (cf. Ly ), can study the full redshift range of reionization.
Diagnostics:
1. When: direct measure of epoch of reionization
2. Process: Inside-out vs. outside-in reionization
3. Sources: Xrays vs. UV vs. shocks
4. Feedback due to galaxy/AGN formation
5. Exotic mechanisms: Very high z particle decay
1e13 Mo
1e9 Mo
HI mass limits => large scale structure
Reionization
Brightness Temperature• Brightness temp = measure of surface brightness (Jy/SR, Jy/beam, Jy/arcsec2)
•TB = temp of equivalent black body, B, with surface brightness = source surface brightness at : I = S / = B= kTB/ 2
• TB = 2 S / 2 k
• TB = physical temperature for optically thick thermal object
• TA <= TB always
Source size > beam TA = TB (2nd law therm.)
Source size < beam TA < TB
beam
source
telescope
TB
[Explains the fact that temperature in focal plane of optical telescope cannot exceed TB of a source]
HI 21cm radiative transfer: large scale structure of the IGM
LSS: Neutral fraction / Cosmic density / Temperature: Spin, CMB
Furlanetto et al. 2006
Spin Temperature
€
n1
n2
= 3e−hν 21 / kTs
1. Collisions w. e- and atoms
2. Ambient photons (predominantly CMB)
3. Ly resonant scattering: Wouthuysen-Field effect = mixing of 1S HF levels through resonant scattering of Ly drives Ts to Tkin
Ly
21cm
Each Ly photon scatters ~ 1e5 times in IGM before redshifting out of freq window.
h21/k = 0.067K
Dark Ages HI 21cm signal
•z > 200: T = TK = Ts due to collisions + Thomson scattering => No signal
•z ~ 30 to 200: TK decouples from T, but collisions keep Ts ~ TK => absorption signal
•z ~ 20 to 30: Density drops Ts~ T => No signal
TK = 0.026(1+z)^2
T = 2.73(1+z)
Furlanetto et al. 2006
Enlightenment and Cosmic Reionization -- first luminous sources
•z ~ 15 to 20: TS couples to TK via Lya scattering, but TK < T => absorption
•z ~ 6 to 15: IGM is heated (Xrays, Lya, shocks), partially ionized => emission
•z < 6: IGM is fully ionized
TK
T
Signal I: Global (‘all sky’) reionization signature
Signal ~ 20mK < 1e-4 sky
Possible higher z absorption signal via Lya coupling of Ts -- TK due to first luminous objects
Feedback in galaxy formation
No Feedback
Furlanetto, Oh, Briggs 06
Signal II: HI 21cm Tomography of IGM Zaldarriaga + 2003
z=12 9 7.6
TB(2’) = 10’s mK
SKA rms(100hr) = 4mK
LOFAR rms (1000hr) = 80mK
Signal III: 3D Power spectrum analysis
SKA
LOFAR
McQuinn + 06
only
+ f(HI)
PS dependence on neutral fraction
xi = 0.13
xi=0.78
z=10
Furlanetto et al. 2006
‘knee’ ~ characteristic bubble scale ~ 1 to 10Mpc (cm)
Inside-out vs. Outside-in
Furlanetto et al. 2004
z=12z=15
Inside-out Outside-in
Sensitivity of MWA for PS measurements (Lidz et al. 2007)
1yr, 30MHz BW, 6MHz chan
Will measure PS variance over k ~ 0.1 to 1 Mpc-1 (cm)
Sensitivity is maximized with compact array configuration (blue)
Sensitivity of MWA for PS measurements (Lidz et al. 2007)
Constrain amplitude of PS (variance) to 5 to 10
Constrain slope of PS to similar accuracy
• N(HI) = 1e13 – 1e15 cm^-2, f(HI/HII) = 1e-5 -- 1e-6 => before reionization N(HI) =1e18 – 1e21 cm^-2
• Lya ~ 1e7 21cm => neutral IGM opaque to Lya, but translucent to 21cm
Signal IV: Cosmic Web after reionization
Ly alpha forest at z=3.6 ( < 10)
Womble 96
z=12 z=819mJy
130MHz
• radio G-P (=1%)
• 21 Forest (10%)
• mini-halos (10%)
• primordial disks (100%)
Signal IV: Cosmic web before reionization: HI 21Forest
• Perhaps easiest to detect (use long baselines)
• ONLY way to study small scale structure during reionization
159MHz
Radio sources beyond the EOR
sifting problem (1/1400 per 20 sq.deg.)
2240 at z > 6
1.4e5 at z > 6
S120 > 6mJy
Signal V: Cosmic Stromgren spheres around z > 6 QSOs
0.5 mJy
LOFAR ‘observation’:
20xf(HI)mK, 15’,1000km/s
=> 0.5 x f(HI) mJy
Pathfinders: Set first hard limits on f(HI) at end of cosmic reionization
Easily rule-out cold IGM (T_s < T_cmb): signal = 360 mK
Wyithe et al. 2006
5Mpc
Dark age HI 21cm signal: baryon oscillations
Barkana & Loeb: “Richest of all cosmological data sets”
• Three dimensional in linear regime
• Probe to k ~ 103 Mpc-1 vs. CMB limit set by photon diffusion ~ 0.2Mpc-1
•Alcock-Pascinsky effect
•Kaiser effect + peculiar velocites
0.1 1.0 10 Mpc-1
Challenge: sensitivity at very low frequency
PS detection
• 1 SKA, 1 yr, 30MHz (z=50), 0.1MHz
•TBsky = 100(/200MHz)-2.7 K
= 1.7e4 K
At l=3000, k=0.3 Mpc-1
• Signal ~ 2 mK
• Noise PS ~ 1 mK
Requires few SKAs
BREAK
Challenge I: Low frequency foreground – hot, confused sky
Eberg 408 MHz Image (Haslam + 1982)
• Coldest regions: T ~ 100z)-2.6 K
• 90% = Galactic foreground
• 10% = Egal. radio sources ~ 1 source deg-2 with S140 > 1 Jy
Signal < 20mK
Sky > 200 K
DNR > 1e4
Solution: spectral decomposition (eg. Morales, Gnedin…)
Foreground = non-thermal = featureless over ~ 100’s MHz
Signal = fine scale structure on scales ~ few MHz
10’ FoV; SKA 1000hrs
Signal/Sky ~ 2e-5
Cygnus A
500MHz 5000MHz
Simply remove low order polynomial or other smooth function?
Cross correlation in frequency, or 3D power spectral analysis: different symmetries in frequency space for signal and foregrounds.
Freq
Signal Foreground
Morales 2003
Cygnus A at WSRT 141 MHz 12deg field (de Bruyn)
Frequency differencing ‘errors’ are ‘well-behaved’ ‘CONTINUUM’ (B=0.5 MHz) ‘LINE’ CHANNEL (10 kHz) - CONT
(Original) peak: 11000 Jy noise 70 mJy
dynamic range ~ 150,000 : 1
10 kHz channels
12o
Cygnus A w. PAPER/GB 130 MHz 30deg FoV
Frequency differencing with MHz channels doesn’t work well for far-out sidelobes due to chromatic aberration.
1MHz separation
5MHz separation
10o
‘stochastic noise term’
Calibration requirements for point source removal
20o x 20o FoV => expect brightest source ~ 34Jy at 158MHz
CSS detection: 10mK signal over 10’ = 73 uJy
required DNR = 34Jy/73uJy = 4.6e5
DNR ~ Nant / (21/2 x rms phase error in rad)
500 element array=> required rms phase error ~ 0.045o
Galactic foreground polarization‘interaction’ with polarized beams
frequency dependent residuals!
Solution: good calibration of polarization response
NGP 350 MHz 6ox6o ~ 5 K pol IF Faraday-thin 40 K at 150 MHz
WENSS: Schnitzeler et al A&A Jan07
30o x 30o
Speed of light in ionosphere is dependent function of ne
Phase (path length) variation proportional to wavelength2
Challenge II: Ionospheric phase errors – varying e- content
Sources wander by ~ arcmin, timescales of mins.
‘Isoplanatic patch’ = few deg = few km
Ionospheric storms wash-out sky completely
74MHz Lane 03
Challenge II: Ionospheric phase errors – varying e- content
TID
Arcmin seeing => problem is not fuzzing-out 21cm signal itself (typically larger scale) , but calibration errors leading to large (freq-dependent) sidelobes from bright sources
Solution
Direction dependent calibration: Wide field ‘rubber screen’ phase self-calibration = ‘peeling’
Virgo A 6 hrs VLA 74 MHz Lane + 02
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15’
Ionospheric phase errors: The Movie
Challenge III: Interference
100 MHz z=13
200 MHz z=6
Solutions -- RFI Mitigation (Ellingson06)
Digital filtering: multi-bit sampling for high dynamic range (>50dB)
Beam nulling/Real-time ‘reference beam’
LOCATION!
Beam nulling -- ASTRON/Dwingeloo (van Ardenne)
Factor 300 reduction in power
VLA-VHF: 180 – 200 MHz Prime focus CSS search Greenhill, Blundell (SAO); Carilli, Perley (NRAO)
Leverage: existing telescopes, IF, correlator, operations
$110K D+D/construction (CfA)
First light: Feb 16, 05
Four element interferometry: May 05
First limits: Winter 06/07
Project abandoned: Digital TV
KNMD Ch 9
150W at 100km
RFI mitigation: location, location location…
100 people km^-2
1 km^-2
0.01 km^-2
(Briggs 2005)
Multiple experiments under-way: ‘pathfinders’
MWA (MIT/CfA/ANU) LOFAR (NL)
21CMA (China) SKA
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Complication: ‘Tile’ diffraction beam
3
EDGE/CORE
EDGES (Bowman & Rogers MIT)
All sky reionization HI experiment. Single broadband dipole experiment with (very) carefully controlled systematics + polynomial baseline subtraction (7th order)
rms = 75 mK
VaTech Dipole Ellingson
300K
150K
Limits to the Global Step in the HI 21cm signal
Currently limited by systematics
After ~1/2 hour, rms no longer decreases as root time
GMRT 230 MHz – HI 21cm abs toward highest z (~5.2) radio AGN
0924-220 z=5.2
S230MHz = 0.5 Jy
1”
8GHz Van Breugel et al.
GMRT at 230 MHz = z21cm
RFI = 20 kiloJy !
CO Klamer +
M(H2) ~ 3e10 Mo
GMRT 230 MHz – HI 21cm abs toward highest z radio AGN (z~5.2)
rms(20km/s) = 5 mJy
229Mhz 0.5 Jy232MHz 30mJy
rms(40km/s) = 3mJy
N(HI) ~ 2e20TS cm-2 ?
Limits:
Few mJy/channel
Few percent in optical depth
Focus: Reionization (power spec,CSS,abs)
C. Carilli, A. Datta (NRAO), J. Aguirre (Penn)
PAPER: Staged Engineering• Broad band sleeve dipole + flaps
• 8 dipole test array in GB (06/07) => 32 station array in WA (2008) to 256 (2009)
• FPGA-based ‘pocket correlator’ from Berkeley wireless lab: easily scale-able
• S/W Imaging, calibration, PS analysis: AIPY + Miriad/AIPS => Python + CASA, including ionospheric ‘peeling’ calibration
100MHz 200MHz
BEE2: 5 FPGAs, 500 Gops/s
CygA 1e4Jy
PAPER/WA -- 4 Ant, July 2007
RMS ~ 1Jy; DNR ~ 1e4
Parsons et al. 2008
1e4Jy
Lunatic fringe: probing the dark ages from the dark side of the MoonC. Carilli (NRAO), Sackler Cosmology Conf, Cambridge, MA, 2008
Long History of Lunar Low Freq Telescope
Gorgolewski 1965: Ionospheric opacity
• Ionosphere p ~ 10 MHz
• ISM p ~ 0.1 MHz
• Interstellar scattering => size ~ 1o (/1 MHz)-2
• Faraday rotation => no polarization
• z > 140 => not (very) relevant for HI 21cm studies, ‘beyond dark ages’
New window
Lunar window
ion. cutoff ~ 30m
ISM cutoff ~ 3km
Return to moon is Presidential directive
Summary of STScI Workshop, Mario Livio, Nov. 2006
“The workshop has identified a few important astrophysical observations that can potentially be carried out from the lunar surface. The two most promising in this respect are:
(i) Low-frequency radio observations from the lunar far side to probe structures in the high redshift (10 < z< 100) universe and the epoch of reionization
(ii) Lunar ranging experiments…”
Our consensus: Lunar imperative awaits lessons from ground-arrays
Advantage I: Interference
Lunar shielding of Earth’s auroral emission at low freq (Radio Astronomy Explorer 1975)
Alexander + 1975
12MHz
ITU radio regulation Article 22: Far-side of Moon is a radio protected by international agreement
Clementine (NRL) star tracker
Advantage II: Very thin ionosphere -- critical at very low frequencieso Tenuous photoionized layer extending to 100km
o p = 0.2 to 1 MHz ~ 0.01 to 0.1 x Earth
o Large day/night variation
Other practical advantages
• Easier deployment: robotic or human
• Easier maintenance (no moving parts)
• Less demanding hardware tolerances
• Very large collecting area, undisturbed for long periods (no weather, no animals, not many people)
Apollo 15
• Array data rates (Tb/s) >> telemetry limits, requiring in situ processing, ie. low power super computing (LOFAR/Blue Gene = 0.15MW)
• RFI shielding: How far around limb is required?
• Thermal cycling (mean): 120 K to 380 K
• Radiation environment
• Regolith: dielectric/magnetic properties
Lunar challenges
Lunar shielding at 60kHz
Takahashi + Woan
Radio astronomy – Probing Cosmic Reionization
•‘Twilight zone’: study of first light limited to near-IR to radio
• First constraints: GP, CMBpol => reionization is complex and extended: zreion = 7 to 15
• HI 21cm: most direct probe of reionization
• Low freq pathfinders: All-sky, PS, CSS
• SKA: direct imaging of IGM
• Lunar array to probe the Dark Ages
END
CSS: Constraints on neutral fraction at z~6
Fan et al 2005